Most adsorbents used for direct air capture of carbon dioxide are reversible. These adsorbents can be cycled several times to capture and release carbon dioxide. The most common chemisorbents used for direct air capture of carbon dioxide from the air include calcium and sodium-based sorbents. Physisorbents like metal-organic frameworks (MOFs), activated carbons, and zeolites have low carbon dioxide selectivity, very small carbon dioxide uptake, and poor performance at low carbon dioxide partial pressures. Chemisorbents are proven to be very efficient for direct air capture of carbon dioxide at extremely low concentrations. This article discusses the aqueous alkaline sorbent and solid sorbent systems for direct air capture.

Aqueous Alkaline Systems

Due to the highly dilute nature of carbon dioxide in the air, capturing carbon dioxide requires chemical sorbents with very strong CO2-binding affinities. A well-known alkaline solution that has high-binding energy with carbon dioxide is aqueous calcium hydroxide.

Whether agitated or passive, calcium hydroxide provides one of the simplest concepts where carbon dioxide can be captured from the air. Calcium carbonate is precipitated and accumulated from the reaction. The calcium carbonate is dried and calcinated to release the captured carbon dioxide as a concentrated stream to produce calcium oxide. Calcium hydroxide is then regenerated by hydrating calcium oxide. The use of calcium hydroxide requires large energy inputs because of sorbent regeneration.

In the use of calcium hydroxide sorbent, the drying and calcination processes give the largest part of the energy penalty. About 179 kJ/mol of carbon dioxide is required to calcinate calcium carbonate whereas only about 109.4 kJ/mol of energy is needed to convert calcium carbonate to calcium oxide.

Also, calcium hydroxide has very low solubility in water, thus limiting the concentration of the hydroxide present to bind carbon dioxide. These disadvantages of calcium hydroxide make it necessary to search for more efficient sorbents.

Caustic solutions such as sodium hydroxide and potassium hydroxide are better alternatives. The use of sodium hydroxide to capture carbon dioxide is due to its strong binding nature for CO2. The efficiency of Sodium hydroxide is similar to calcium hydroxide. However, the former produces sodium carbonate when reacted with carbon dioxide, which has a high solubility in water. The sodium carbonate formed is reacted with calcium hydroxide to produce calcium carbonate. Sodium hydroxide is also regenerated in the process. The calcium carbonate formed is calcinated to produce calcium oxide and carbon dioxide. The carbon dioxide is transferred and compressed.

Sodium hydroxide still shares the same energy disadvantage as calcium hydroxide. Potassium hydroxide can be used to capture carbon dioxide, but it is more expensive than sodium hydroxide. Due to the large energy penalties for aqueous alkaline solutions, energy and economic evaluations may require baseline designs.

The aqueous alkaline adsorbent system is industrially practiced by Carbon Engineering, a Canadian company. Carbon Engineering uses potassium hydroxide solution as the adsorbent. The potassium carbonate solution resulting from the adsorption of carbon dioxide by the aqueous potassium hydroxide is reacted with calcium oxide. Calcium carbonate is formed, which is calcinated to release carbon dioxide and give calcium oxide.

In industries, the commonly used method to sorb a gas into a solution is a tower filled with packing materials. The solution is allowed to drip down the tower while the gas is blown up from the tower bottom. A few recent studies have reported the use of short columns with very large cross-sections due to the dilute nature of carbon dioxide.

Some recent studies have evaluated the feasibility of spray towers instead of packed towers as the system component that contacts the air. Spray towers provide large surface areas for air-liquid contact and allow for the maintenance of low-pressure drops. Although the spray tower avoids the cost of large, packed towers, it incurs energy losses from the spray.

Solid Sorbent Systems

In solid sorbent systems, carbon dioxide is absorbed on the surface of the sorbents. Only a few studies have been done on the use of solid inorganic bases, instead of solutions, to capture ultra-dilute carbon dioxide from the air.

A recent study carried out a thermogravimetric, kinetic, and thermodynamic analysis of the carbonation rates of calcium hydroxide and calcium oxide. The study measured moisture effects on the carbonation rate in the absence of water.

The carbon dioxide uptake is very fast during the first few minutes at carbonation temperatures between 300 ℃ and 450 ℃ for calcium oxide. A recent study reported an initial 44% carbon dioxide uptake from a 500-ppm carbon dioxide concentration. At temperatures below 300 ℃, no carbonation takes place. At temperatures higher than 450 ℃, calcium carbonate decomposes into calcium oxide and carbon dioxide. Water vapor increases the degree of carbonation by about 80%.

Unlike calcium oxide, the carbonation of calcium hydroxide is faster with a higher conversion degree. It is faster in the presence or absence of water. Also, calcium hydroxide carbonation is effective even at lower carbonation temperatures (200 ℃ to 425 ℃).

Another study investigated the application of solar power to achieve carbonation and calcination. The researchers exposed a solar reactor which contains a fluidized bed of reacting particles directly to high-flux solar radiation. This provided efficient heat and mass transfer and eliminated the need to transport the solids. The carbonation of calcium hydroxide occurs between 800 ℃ and 875 ℃ using the solar reactor. Calcium oxide is carbonated between 365 ℃ and 400 ℃. Water vapor increases the carbonation kinetics of carbon dioxide capture.

The energy input for solid sorbent systems is significantly higher than that of aqueous alkaline solutions. The thermal energy input for completing the calcium oxide to calcium carbonate cycle is reported to be around 10.6 MJ/mol of carbon dioxide. That is, if calcination occurs at 875 ℃ and carbonation occurs at 375 ℃, the additional energy needed to heat ambient air to 375 ℃ before carbonation accounts for the high energy input.

Amine compounds have been used as solid adsorbents by several companies to capture carbon dioxide from the air. Climeworks AG captures carbon dioxide from moist air by making use of amine compounds bonded to dry porous granulates as filter materials. The company regenerates the adsorbent once it is fully enriched with carbon dioxide by combining temperature swing adoption (TSA) and vacuum swing adoption (VSA) at low temperatures of approximately 100 ℃. Climeworks delivers carbon dioxide at 99% purity.

Global Thermostat, a U.S. company operates with the adsorption and desorption principles. The company uses porous honeycomb ceramic monoliths as the filter material. The amine compounds are deposited on the surface of the filter material. Global Thermostat regenerates the adsorbent using low-temperature steam at 85 ℃ to 100 ℃. It delivers carbon dioxide at 98% purity.

For sodium-based solid sorbents, the carbonation temperature varies significantly. The carbonation of sodium bicarbonate reaches completion between 90 ℃ and 200 ℃ within 3 minutes based on the number of cycles. Sodium carbonate reaches carbonation completion between 1000 ℃ and 1400 ℃ within 15 minutes.

A recent study on the thermogravimetric analysis of sodium-based solid sorbents showed significantly slow carbonation reaction rates. Also, the large rates of mass flow render the carbonation process of sodium-based solid sorbents less efficient.

Future perspective

Since the introduction of direct air capture in 1999, many studies have focused on the use of different sorbents. However, much progress has been made only with chemisorbents for capturing carbon dioxide from the air.

Future research directions can focus on improving the efficiency of the sorbents and carbon dioxide diffusion. There are opportunities to develop new liquid and/or solid sorbents to capture carbon dioxide from the air.

Further, the structure-property relationships of sorbents that will provide the basis for rational designs of carbon dioxide capture sorbents need to be studied. To achieve an economically feasible process for effective gas/sorbent contacting strategies, sorbents should be developed for commercial-scale direct air capture.